Containers have long been employed to store and transfer food prior to presenting the food at a market where it will be purchased by the consumer. After meats, fruits, and vegetables are harvested, they are placed into containers to preserve those foods for as long as possible. Maximizing the time in which these foods remain preserved in the containers increases the profitability of all entities in the chain of distribution by minimizing the amount of spoilage.
The environment around which the foods are preserved is the most critical factor in the preservation process. Not only is maintaining an adequate temperature important, but the molecular content of the gases surrounding these foods is significant as well. By providing an appropriate gas content to the environment surrounding the food, the food can be better preserved when maintained at the proper temperature or even when it is exposed to variations in temperature. This gives the food producer some assurance that after the food leaves his or her control, the food will be in an acceptable condition when it reaches the consumer.
Each type of food has an optimum gas concentration in which it is best preserved. For example, fish and crustaceans are much better preserved when exposed to high levels of carbon dioxide (CO.sub.2) such as 60% to 80%. On the hand, beef turns brown in the absence of oxygen (O.sub.2) and the proper mixture is approximately 80% O.sub.2 and 20% CO.sub.2. Alternatively, poultry preserves best when exposed to nitrogen (N.sub.2) and carbon dioxide with the ideal concentration being approximately 75% N.sub.2 and 25% CO.sub.2.
With respect to fruits and vegetables, the spoilage process is quite different than for meats because fruits and vegetables remain alive after harvesting. Fruits and vegetables undergo a process known as respiration in which they take in oxygen and give off heat energy, carbon dioxide, water vapor, and occasionally ethylene. Each species has a different respiration rate. The respiration rate is also affected by external factors, namely, the carbon dioxide concentration, the oxygen concentration, the temperature, and the ethylene concentration. Generally, a species' tolerance to spoilage at typical storage temperatures is enhanced by maintaining oxygen levels above 5% while maintaining carbon dioxide levels below 20%. However, it is also desirable to keep aerobic bacteria from growing and multiplying which is accomplished by maintaining a lower oxygen level. But anaerobic bacteria, such as Clostridium botulinim, will grow if no oxygen is present. As such, the balance between these competing factors typically results in a concentration of oxygen of less than 10% but greater than 5% for most fruits and vegetables. The remainder of the gas is nitrogen until respiration occurs which results in the addition of carbon dioxide, ethylene, and water vapor. To limit respiration and prevent rapid spoilage, it is desirable to continuously modify the gaseous environment surrounding the food by replenishing the supply of oxygen which is consumed and removing the byproducts which are produced during respiration.
To assist in the transmission of oxygen into the container and in the removal of carbon dioxide, ethylene, and water vapor from the container, permeable polymer films, or membranes, have been employed. In some situations, it is best to use a membrane with a high permeability to gases so that those gases can be readily transferred into and from the container. In other situations, it is best to maintain the initial environmental gas concentration, such as when meats are packaged, which can be done by use of a membrane with a low permeability. Generally, the rate at which a specific gas permeates through a membrane is proportional to the difference between the concentrations of that specific gas on both sides of the permeable membrane. If there is 0% carbon dioxide on one side of the membrane and a high concentration of carbon dioxide on the other, permeation would be high. On the other hand, if air with 20% oxygen is on both sides of the membrane, permeation would be low.
The permeation rate from a container is proportional to the surface area of the permeable membrane. So to ensure that the appropriate permeation is accomplished, the surface area cannot be obstructed. Otherwise, permeation from the surface will not occur. As can be expected, this problem is often encountered during storage and shipping in which numerous containers having these permeable film membranes are located adjacent each other. When the containers are stacked, the problem is accentuated as the likelihood that a portion of the permeable membrane will be obstructed vastly increases.
Considering that heat is also a byproduct of the respiration process and maintaining lower temperatures is desirable, some fruits and vegetables such as strawberries require the heat to be dissipated. If not, then the increased temperature will cause increased respiration resulting in a "snowball" effect and a quickly spoiled product. In these situations, the use of a contained environment augmented by a permeable membrane is not advantageous since such a configuration would tend to contain the heat. Instead, no membrane is used in this type of package and additional vents are provided to allow unimpeded access of cool gas around the product. However, when these packages are stacked vertically to use less space in storage and transportation, the vent holes can be obstructed due to the stacking configuration. Attempts have been made to align the vents on the base of one container to the lid of another to keep a free flow of air between adjacent containers and dissipate the heat. However, as the heat rises from the lowest stacked container into the vertically adjacent container, it raises the temperature in that container as well. As the warm air continues to rise from package to package, the heat increases such that the temperature of the air around the food in the top package in the stack can become unacceptably high.
Attempts have also been made to place vents on the side of the tray. But, the addition of any openings on the tray can comprise the structural integrity of the package. And since the vast majority of containers today are made of less costly, thin polymers, the strength issue is a major concern. Furthermore, additional openings along the side of the package makes the enclosed food more susceptible to exposure to moisture, dirt, insects and the like during storage and transportation.
As the tastes of consumers continue to transition from canned and frozen foods to fresh foods, the need for improved containers is growing. Such an improved container must overcome the aforementioned shortcomings associated with occlusion of the surface of the permeable membrane and maintaining the appropriate environment during stacking.